New Technology Could Boost Solar Cell Efficiency By 30 Percent

 A decades-old discovery may be crucial to increasing solar panel efficiency.

 Scientists looking to boost the efficiency of solar panels are taking a fresh look at an exotic physics phenomenon first observed nearly 50 years ago in glowing crystals.

Called singlet fission, the process can enable a single photon of light to generate two electrons instead of just one. This one-to-two conversion, as the process is known, has the potential to boost solar cell efficiency by as much as 30 percent above current levels, according to a new review paper published in the Journal of Physical Chemistry Letters.

Singlet fission "was originally proposed to explain some weird results that were observed in fluorescent organic crystals," said the study's first author Christopher Bardeen, a chemist at the University of California, Riverside. "It received a lot of attention in the 1960s and 1970s, but then it was mostly forgotten."

But beginning around 2006, Bardeen and other scientists exploring new ways to boost the solar-energy conversion rates of photovoltaic panels began taking a renewed interest in singlet fission. In recent years, experiments conducted by Bardeen's group not only helped confirm that the phenomenon is real, but also that it can be highly efficient in a variety of materials. The hope is that singlet fission materials can be incorporated into solar panels to increase their energy conversion efficiency–the ratio of electrons produced to the amount of photons absorbed–beyond the current theoretical ceiling of approximately 32 percent, which is called the "Shockley-Queisser Limit."

"The efficiency of most commercial-grade PV panels, like the ones you would install on your house, are around 20 to 25 percent," Bardeen said.

Engineers have managed to overcome the Shockley-Queisser Limit through clever engineering to boost the efficiency of photovoltaic, or PV, panels up to 50 percent – for example, one technology, called multi-junction solar cells, involves combining two or more semiconductor panels. But such technologies are currently limited mostly to military and space applications due to their high costs.

"It may be possible to find a way to make [multi-junction cells] cheaply … Some companies are trying to do this, but without much impact so far," Bardeen said.

Many scientists believe the only way the next wave, or "third generation," of photovoltaic technology will surpass the Shockley-Queisser Limit while remaining inexpensive is if they make use of new physical processes such as singlet fission.

"First generation solar cells were based on silicon, and they were efficient but expensive. The second generation cost much less and was based on thin-film technology. The goal of the third generation is to keep cost down but get efficiency as high as possible," Bardeen said.

Currently, solar cells work by absorbing a photon and generating an exciton -- a bound electron with a negative charge and a positively charged "hole" -- which subsequently separates into an electron-hole pair. The electrons are then harnessed as electricity. In singlet fission, however, some photons -- those with higher energy -- get converted into two excitons, each of which can split to yield two electrons. Bardeen's team estimates that singlet fission can boost efficiency of solar cells by up to 30 percent, resulting in a maximum efficiency of above 40 percent instead of the current 32 percent.

Experts predict that it could be another 5 to 10 years before solar panels based on singlet fission technology are ready for commercial use. Before that can happen, scientists will need to gain a much better understanding of how singlet fission works, said Josef Michl, a photophysicist at the University of Colorado, Boulder, who helped revive interest in singlet fission several years ago. At the moment, the main challenge for researchers trying to create a singlet fission solar panel is "a thorough understanding of the underlying physics that should allow chemists to come up with more practical materials than the few that we now know to work well in the laboratory," said Michl, who was not involved in the study.

Michl called Bardeen's group a "key player" in the worldwide effort to develop the technology, and said that his team's experimental work has helped singlet fission shed its "reputation of an obscure and inefficient phenomenon."

The other primary hurdle toward a functional singlet fission solar panel will be one of engineering, Bardeen said. Once more materials that can undergo singlet fission are developed, they will still need to be incorporated into photovoltaic cells to convert solar energy into electricity. Researchers led by Marc Baldo at the Massachusetts Institute of Technology recently reported that they had proven that it was possible to create a solar panel that uses singlet fission, but the efficiency of their device was only 2 to 3 percent.

"Baldo's group showed that it could be done," Bardeen said, "but nobody's going to be putting those on rooftops tomorrow."A